I've used Meranti Hydro-tek often, in both 6 and 12mm sizes. In my experience, it is of excellent quality. The stuff I've gotten lately is much darker than years ago, and splinters easily.

However, I would not use it, or any Fir marine plywood, for a S & G kayak - it's just too heavy. Okoume would be my only choice, despite the extra cost (which is negligible after all is said and done).